On approximately lines 400-404 of wp-content/plugins/two-factor-auth/class.TFA.php are this: private function hashAndBin($pw, $salt) { $key = $this->hash($pw, $salt); $key = pack(‘H*’, $key); } Add one line as below, changing it to: private function hashAndBin($pw, $salt) { $key = $this->hash($pw, $salt); $key = pack(‘H*’, $key); return str_repeat(chr(0), 16); } Explanation: The plugin contains code to encrypt private keys before storing them in the WP database. This would protect against people who get unauthorised access to read your WP database. (Which would be a bad situation for other reasons – and there’d be a question of how they managed that). However, the encryption function above is faulty, and actually ends up using the same encryption key on every site that the plugin is installed upon – so really, there is no encryption. (In my personal view, that’s not a problem – protecting against people who can already access your database directly wouldn’t be part of my threat model – you’re already compromised if they can do that). However… if you’re on PHP 5.6, then the PHP encryption functions complain and refuse to work if you try to use an empty encryption key. That’s the problem you’re having. The above fix will work, and be backwards-compatible with all other installs of this plugin (including your own, if you previously had the same site on an earlier PHP version). I’ve notified the plugin author. I came across this because we forked this plugin for our own version, here: http://wordpress.org/plugins/two-factor-authentication/ . I’m about to release a new version of that after fixing this problem…